Twist drills

ABSTRACT

A twist drill having a point angle within the range of 55* to 73*, a helix angle within the range of 10* to 20*, and a web thickness within the range of 18.5 to 25 percent of the drill diameter.

United States Patent [1 1 Hibbert et al. [451 Jan. 9, 1973- [54] TWISTDRILLS [58] Field of Search ..408/230, 229, 228, 227

[75] Inventors: Charles l-libbert, Sheffield;

Frederick Arthur Kirk, Moorgate; [56] References cued Keith Siddall,Gleadless; Michael UNITED STATES PATENTS Roi Howard, Brentwood, all ofEngland 2,859,645 11/1958 Emmons et a] ..408/23O 2,328,629 9/1943 Eichet al. [73] Assignee: Osborn-Mushet Tools Limited, Shef- 750,537 1/ 1904Hanson ..408/230 field, England Primary ExaminerGil Weidenfeld [22]Ffled' 1970 Attorney-Lackenbach & Lackenbach [21] Appl. N0.: 81,903

[57] ABSTRACT [30] Foreign Application Priority Data A twist drillhaving a point angle within the range of O 2 6 55 to 73, a helix anglewithin the range of 10 to 20, Ct. 0,19 9 GlBflt Blltaln ..5l,368/69 anda web thickness Within the range f to U S Cl 8/ 9 percent of the drilldiameter. [51] Int. Cl. ..B23b 51/02 12 Claims, 2 Drawing FiguresTWISTDRILLS This invention relates to twist drills, in particular butnot exclusively to drills intended for drilling grey cast iron, and hasfor its object to provide improvements therein.

According to the invention there is provided a twist drill having apoint angle within the range of 55 to 73, a helix angle within the rangeof to 20, and a web thickness within the range of 18.5 to 25 percent ofthe drill diameter. Desirably the point angle is within the range of 58to 62, or better still the point angle is 60..

Desirably the helix angle is within the range of 13 to 18, or betterstill within the range of 14 to 16, but preferably the helix.angle isDesirably the web thickness is within the range of 19 to 24 percent, orbetter still within the range of 20 to 22 percent, but preferably it is21 percent of the drill diameter. Desirably the drill is made of a steelof which the composition falls within the ranges specified in line I ofthe table set out below or better still within any of the rangesspecified in lines 11 to IV of the table, but preferably is of any ofthe compositions specified in lines V to VII of the table (thatspecified in line VII being at present thought best and in line VInearly as good). Desirably the steel may also be of a composition asspecified in any one oflines VIII to XII of the table.

i.e. the thickness of the drill between the flutes as shown at 22 is0.0525 inch with a tolerance of: 0.0025 inch; thus it represents 21percent i 1 percent (i.e. 20 to 22 percent) of the drill diameter. Thetip 14 is formed in customary manner with a chisel point 22 (see FIG. 2)and this is set at an angle of 125 to the leading edge of the lands 18;the chisel edge angle is indicated at 24.

The drill is made from steel having the composition specified in lineVII of the table set out above. This will have been subjected to a formof consumable electrode remelting namely electro-slag refining.

"xiitiirrhrof drill is identical to that described above save that it ismade from steel of the composition specified in line VI of the table.

m twist drills described above, it is thought, will perform particularlywell for the purpose of drilling grey cast iron and exhibit a lifebetween required regrinding far in excess of presently used twist drillsfor this purpose.

It will be appreciated that modifications may be made from theparticular forms of drill described above without departing from thescope of the invention. In particular although the point angle isdesirably 60 it may be anywhere within the range of 55 to 73 butdesirably within the range 58 to 62. Similarly although in the preferredembodiment the helix angle is 1n the table, the figures F'prsen'tpercentages by In order that the invention may be clearly un derstoodand readily carried into effect, the same will now be described, by wayof example only, with reference to the accompanying drawings in which:-

Fl'G. 1 shows a side elevation of V4 inch twist drill, and,

FIG. 2 is an end view of its point.

The drill shown in the drawing is of V inch diameter and 3% inches inoverall length, and has two flutes 10 extending along 1% inches of itslength, i.e., along 50 percent or less of its length. Thus the drill maybe regarded as a stub drill. The drill has a point generally designated12 formed by a tip 14 and two flanks 16. The point angle, i.e. the anglesubtended by the flanks '16 at the tip 14 is 60. The flutes 10 areformed with cylindrical lands 18 and these describe a helix with respectto the axis of the drill; the helix angle, i.e'. the angle between thelands l8 and a diametrical plane of the drill is 15 as indicated at 20.The web thickness,

15 it may lie anywhere within the range of 13 to 18 but desirably withinthe range of 14 to 16. The web thickness could lie anywhere within therange 19 to 24 percent of the drill diameter although as previouslyindicated it is preferred that it should lie within the range of 20 to22 percent and ideally should be 21 percent of the drill diameter wherethe drill diameter is up to 1% inches in diameter. In drills 1% inchesor larger, the web thickness is desirably proportionally somewhat lessdown to the lower limit previously referred to, viz: 19 percent of thedrill diameter. Furthermore, a drill embodying the invention may be madefrom a grade of high speed steel other than of the compositionsdescribed above.

What we claim and desire to secure by Letters Patent is:

l. A twist drill particularly adapted for drilling grey cast iron havinga point angle within the range of 55 to 73, a helix angle within therange of 10 to 20, and a web thickness within the range of 18.5 to 25percent of the drill diameter.

2. A twist drill according to claim 1, in which the point angle iswithin the range of 58 to 62.

3. A twist drill according to claim 1, in which the helix angle iswithin the range of 1 3 to 18.

4. A twist drill according to claim 1 in which the helix angle is withinthe range of 14 to 16.

3 4 5. A twist drill according to claim 1, in which the web 9. A twistdrill according to claim 2 in which the helix thickness is within therange of 20 to 22 percent of the a le i approximately 15. drilldiameief- 10. A twist drill according to claim 1 in which the 6. A twistdrill according to claim 2 in which the helix point angle isapproximately angle is within the range of 13 to l8. 7. A twist drillaccording to claim 2 in which the web thickness is within the range of20 to 22 percent of the drill diameter.

8. A twist drill according to claim 1 in which the helix angle isapproximately 15. 10

11. A twist drill according to claim 10 in which the helix angle isapproximately 12 A twist drill according to claim 11 in which the drillcomprises a high speed steel.

1. A twist drill particularly adapted for drilling grey cast iron havinga point angle within the range of 55* to 73*, a helix angle within therange of 10* to 20*, and a web thickness within the range of 18.5 to 25percent of the drill diameter.
 2. A twist drill according to claim 1, inwhich the point angle is within the range of 58* to 62*.
 3. A twistdrill according to claim 1, in which the helix angle is within the rangeof 13* to 18* .
 4. A twist drill according to claim 1 in which the helixangle is within the range of 14* to 16*.
 5. A twist drill according toclaim 1, in which the web thickness is within the range of 20 to 22percent of the drill diameter.
 6. A twist drill according to claim 2 inwhich the helix angle is within the range of 13* to 18*.
 7. A twistdrill according to claim 2 in which the web thickness is within therange of 20 to 22 percent of the drill diameter.
 8. A twist drillaccording to claim 1 in which the helix angle is approximately 15*.
 9. Atwist drill according to claim 2 in which the helix angle isapproximately 15*.
 10. A twist drill according to claim 1 in which thepoint angle is approximately 60*.
 11. A twist drill according to claim10 in which the helix angle is approximately 15*.
 12. A twist drillaccording to claim 11 in which the drill comprises a high speed steel.